1. Field of the Invention
This invention relates to a method for the combined and preferably wafer-based housing of micromechanical systems comprising individually different operating pressures. Using a single operation, this method enables to selectively fill one cavity 1 with defined gas pressure P1 and one cavity 2 with gas pressure P2, in which P1 and P2 can be chosen independently. This method allows to combine different micromechanical systems into one component. The degree of integration of these systems is therefore increased substantially. In addition, this invention relates to micromechanical systems comprising at least two cavities and different internal pressures and/or different gas compositions in each cavity.
2. Description of Related Art
For quite some time, components produced by microsystem technology (micro electro-mechanical systems, MEMS) have allowed the miniaturized and cost-effective production of sensors and actuators. Microsystem technology (MST) is a rather new branch of technology which employs the silicon-based and highly effective production processes used in semiconductor industries in order to transfer macroscopic technology systems into microscopic structures. This supports the continued miniaturization and performance enhancement of technical products. MST products are used in many industries, such as microelectronics, industrial automation, communication technology, medical technology, within the automobile industries and for life science products. Advances in miniaturization and the continued increase in integration density of microsystems necessitate the ongoing development and improvements of existing production methods.
The automobile industry and engineering need complex, integrated microsystems components in order to perform a multitude of measuring and controlling functions, autonomously and with a minimum of energy. Depending on their requirements, different sensor systems need different operating pressures. Resonant systems frequently require high performance implementation. Any mechanical damping through ambient gas in the cavity in which the sensor system is placed thus needs to be minimized by keeping the cavity operating pressure low. For example, resonant rotary rate sensors are typically operated at pressures of one μbar to a few mbar.
On the other hand, accelerometers have to be damped strongly and their operating pressure is typically at a few hundred mbar. The following table shows the typical operating pressure of different microsystems:
Sensor/Component TypeOperating PressureAccelerometer300-700mbarAbsolute pressure sensor1-10mbarResonant sensor (e.g., rotary rate0.1mbarsensor)Bolometer<0.0001mbarRF Switch<0.0001mbar
FIG. 2 shows the typical assembly of a resonant inertial sensor produced using microsystem technology (P. Merz, W. Reinert, K. Reimer, B. Wagner, ‘PSM-X2: Polysilicon surface micromachining process platform for vacuum-packaged sensors, Konferenzband Mikrosystemtechnik-Kongress 2005, D/Freiburg, VDE Verlag, p. 467-470). The micromechanical bottom surface sensor contains the active sensor structure (MEMS active layer). Free standing structures are formed through a specific etch process which removes a sacrificial layer. Out-of-plane movements are detected capacitively by mounting counter-electrodes at a distance of 1.5 μm. Therefore, the direction of movement of micromechanical systems is not limited to in-plane movements, out-of-plane movements can also be generated and detected. Inside the upper chip (the cap), a 60 μm deep cavity contains getter material used to absorb and chemically bind gas molecules. The solid bonding of sensor and cap wafer is accomplished at the wafer level through a gold-silicon eutectic (so-called Wafer-Level-Packaging). The bonding frame of gold-silicon provides a hermetic seal which retains the pressure set during the eutectic bonding procedure. The getter layer deposited into the cavity ensures that a minimal cavity pressure of as low as 1E-6 bar can be set and maintained throughout the life of the component.
The housing of micro-sensors is one of the least developed yet one of the most important and most challenging technology fields within the art of microsystem technology. A key technology for many micromechanical components is the provision of a hermetic housing. A hermetically sealed encapsulation protects micro-sensors against harmful environmental agents (dust, mechanical and chemical damages) and prolongs the reliable function and life of the sensor. In addition, modern resonant operated micro-sensors require a specific operating gas or a defined ambient pressure within the housing cavity in order to function properly.
The so-called Wafer-Level-Packaging (WLP) enables the encapsulation of open sensors at the wafer level. This is accomplished by providing a cap wafer containing the individual functional elements of the housing. The cap wafer is assembled with the sensor wafer such that each sensor chip is solidly bounded to a housing chip. Only after this assembly at the wafer level is the wafer pair then individualized into single chips. Through its massively parallel production, the housing at the wafer level provides enormous advantages with respect to costs, component integration density and yield compared to housing at the chip level.
A number of well established methods are available for use in WLP technology, such as glass frit bonding, anodized wafer bonding, direct bonding (fusion bonding), eutectic bonding, thermo-compression bonding, adhesive bonding or gluing (see R. F. Wolffenbuttel, K. D. Wise, ‘Low-temperature silicon-to-wafer bonding using gold at eutectit temperature’, Sensors and Actuators A, 43, 1994, p. 223-229; M. Madou, ‘Fundamentals of Microfabrication’, CRC Press, Boca Raton, 2002).
Using housing at the wafer level, the production chamber gas and the production pressure are sealed within the cavity. This allows production uniformity in that all components of the wafer can receive an identical cavity pressure. The cavity can be provisioned with atmospheric pressure, subatmospheric pressure and can be overpressurized. Typically, the WLP technology described above can achieve minimal cavity pressures of between 1-10 mbar. Operating pressures below that are normally not achievable. A remaining partial pressure in the range of 1 to 10 mbar will be present through outgassing of materials, surface desorption of molecules and the decomposition of contamination particles.
In order to achieve even lower pressures of less than 1 mbar, additional functional layers called getter layers need to be used which will absorb specific gas molecules (see M. Moraja, M. Amiotti, R. C. Kullberg, ‘New getter configuration at wafer level for assuring long term stability of MEMS’, Proc. of SPIE, Vol. 4980., 2003, p. 260-267; D. Sparks, S. Massoud-Ansari, N. Najafi, ‘Reliable vacuum packaging using Nanogetters™ and glass frit bonding’, Reliability, Testing and Characterisation of MEMS/MOEMS III, Proc. of SPIE, Vol. 5343, 2004, p. 70-78). This can be accomplished through surface adsorption, through volume solubility or through chemical bonding.
A large number of getter materials have been developed within the last few decades. Among those that have been used for quite some time are getters made from metals or alloys such as Ba, Al, Ti, Zr, V, Fe and similar. These have found application in cathode ray tubes, flat screens, particle accelerators and in semiconductor production equipment, for examples see U.S. Pat. Nos. 4,269,624, 5,320,496, 4,977,035 or 6,236,156. These materials absorb or adsorb different gases either through simple surface adsorption or through oxide and hydride formation. So-called non-evaporable getters (NEGs) have been used since the middle of the 1990s, either as tablets or as strips in specially designated recesses or in a ceramic encapsulation in the vicinity of the chip. NEGs have been produced frequently using the methods of powder metallurgy in order to keep the surface area as large as possible. With these methods, the sintering of metal particles is barely initiated so that small gaps remain between the metal pellets. By applying a temperature activation process in vacuum or in a reducing hydrogen atmosphere, the surface oxidation layer which formed during the sintering process is removed. Then, the activation process is completed either by continued heating of the surrounding structure or by electric resistance heating (using an Ohm heater element).